This program of research is aimed at providing fundamental data of both basic and clinical significance for the development and application of the cochlear prothesis. The program is proposed as 7 closely integrated projects involving studies in animals and man to determine the hearing benefits provided by the prothesis, to define the mechanisms of prosthesis function and determine the safe limits of stimulation and the mechanisms that underlie implant use. Project I is an animal psychophysical study to define the relationship between hearing capabilities provided by the implant and parameters of intra and extracochlear stimulation. Two important goals of this study will be to define the relation between hearing and neural survival and to evaluate the feasibility of a multichannel extracochlear electrode system. Project II is a collection of physiological studies to assess response properties of cochlear and brainstem elements to specific electrical waveforms and, with results for other projects (I, III, and VI) to formulate a model of peripheral encoding of electrical signals. Project III is a biophysical project aimed at defining the impedances of cochlear tissues in order to assess current paths generated by the implant and to develop a finite element model of the inner ear. Project IVa and b is a set of histological studies with goals of assessing the safe limits of stimulation, the mechanisms that underlie stimulation induced damage, the development of a multi-electrode extracochlear implant, the evaluation of changes in 8th N-cochlear nucleus synapses and VIII nerve proteins with auditory deprivation and stimulation, and the analysis of CNS metabolic activity changes elicited by electrical stimulation. Project IVc is a histological study with goals of defining the effects of longterm, controlled stimulation in the implanted primate, the effects of early deprivation and implantation in the monkey and the influence of reimplantation procedures on the primate inner ear. Project V is a human study to assess the influence of history of deafness, vibrotactile performance, and perceptual and physiological responsiveness of the system to round window stimulation on subsequent implant performance and speech recognition, and to determine the influence of varying channel number on performance in an optimized multichannel prosthesis. Project VI is a human psychophysical project aimed at assessing the influence of pulsatile, analog and envelope waveform changes on implant performance and to develop an acoustic analog model of electrical hearing.